This invention relates to a blown film process for producing latent elastic films with high cross-direction orientation.
Thermoplastic elastomers and their blends and compounds have inherent set characteristics, resulting in xe2x80x9clatentxe2x80x9d elastic behavior. Elasticity retrieval is most commonly performed by heat shrinking, although several other methods may be employed, including chemical and microwave treatment. Traditionally, the latent character of thermoplastic elastomers is introduced by stretch-orienting under cold or tepid conditions, annealing, and quenching to preserve the fixed xe2x80x9cstrainedxe2x80x9d state of the film. The orientation temperature profile is chosen to deliver certain shrink material mechanical properties, such as shrink tension and shrink percentage. Stretching may be performed uni-, bi- or omni-directionally. Whether stretch orientation is performed in-line with the extrudate production or subsequent to it, the orientation temperature profile is determined by the thermal transitions exhibited by the specific polymer, blend or formulation. Generally accepted analysis methods for determining these various transition temperatures include Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA). These transitions are generally known by those skilled in the art to coincide with crystallites induced by mechanical strain or crystalline regions from the ensuing polymer chemistry.
In the general processing scheme for blown film, particularly single stage blown film, the temperature is not specifically regulated or optimized along the profile of strain-induced orientation. Furthermore, film blowing involves a complex set of interactions between the melt rheology of the extrudate, heat transfer in the changeover from the viscous melt to solid state, and the extensional properties influencing the morphology formation of the film. If the temperature is not properly regulated, premature film breakage causing low yields, bubble instability resulting in poor tension control, and non-uniform gauge and variable through-roll morphology profile are some of the problems encountered in film production. Additionally, due to the elastic nature of these films, bi-directional stretching is further complicated by the inherent nature of the film to spring back during stretch orientation. Furthermore, in the control of elastic material in second stage or post-stretching processes that utilize tenter frames or differential speed, biaxial calender rolls are particularly problematic.
There is thus a need or desire for a method of producing latent elastic films that alleviates the aforementioned problems.
There is a further need or desire for a latent elastic material whereby substantial levels of latent set can be introduced to achieve high shrinkability without compromising the need for low tension set in the post-shrunk material.
The present invention is directed to a blown film process for producing latent elastic films with high cross-direction orientation. In this process, the temperature gradient is preferentially controlled along the expansion length. The regulation of temperature along the bubble profile allows more effective xe2x80x9cdown gaugingxe2x80x9d of the film and improved melt strength of the hot film.
In one embodiment of this invention, a multiple bubble process is employed. In this process a film is blown to minimally orient and sufficiently down gauge the film at elevated temperatures above the melting point. The film is then collapsed. The film is then re-blown at pre-determined stretch, annealing and quench temperatures specified along the bubble expansion path, while simultaneously substantially orienting and completing the final down gauging of the film.
In another embodiment of this invention, a single bubble process is employed. In this process a multi-ring system, an internal bubble cooling system, an internal cooling system, and/or an external air cooling system is used to control the bubble expansion and temperature profile during bubble expansion as the film is down gauged, oriented and set.
The blown film method of the invention applies to both single and multi-component, co-extruded films. Adequate melt strength to maintain the integrity of the film while simultaneously orienting and thinning the film are important process elements in producing cross-direction-oriented, low gauge, latent elastic films.
With the foregoing in mind, it is a feature and advantage of the invention to provide a method of making cross-direction-oriented, low gauge, latent elastic films wherein premature breakage and skin layer separation is minimized and/or eliminated.
This and other features and advantages will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the drawings.